1. Field of the Invention
The present invention relates to a pipe joining method using shape memory alloy, and in particular to a method for joining pipes with a shape memory alloy joint and a method for joining shape memory alloy pipes with a general metal joint.
2. Description of the Prior Art
Welding techniques have been largely used for joining pipes until now, however, due to welding defect, it is regarded preferable to use a mechanical method for connecting or jointing in joining pipes. Because shape memory alloy can be easily fabricated as a pipe joint and there is no defect occurrence, recently shape memory alloy is being watched with keen interest as a pipe joint.
The conventional pipe connecting method using shape memory alloy will be described with reference to accompanying FIGS. 1axcx9c1c. For example, for joining two pipes 4 having an outer diameter of 100 mm, a shape memory alloy joint 2 having an inner diameter of 97 mm is prepared as shown in FIG. 1a. The shape memory alloy joint 2 is expanded below transformation temperature so as to have the inner diameter not less than 100 mm as shown in FIG. 1b. The enlarged shape memory alloy joint 2 is installed at the outer circumference of the pipes 4, heated at a temperature not less than the transformation temperature as shown in FIG. 1c. By a shape memory effect, the shape memory alloy joint 2 tends to return the original shape having an inner diameter of 97 mm, however, it can not return because of the pipes 4 and joins them strongly. In more detail, it is the same that a joint having an inner diameter of 97 mm is forcibly installed at the outer diameter of a pipe having an outer diameter of 100 mm, and accordingly the joint provides a strong joining force.
However, in general, because a pipe has a tolerance, there are problems to be considered. When a tolerance is 2%, although a nominal outer diameter is 100 mm, an outer diameter is actually 98xcx9c102 mm. In that case, in order to insert the pipe, an enlarged joint has to have an inner diameter not less than 102 mm. When the joint is heated at a temperature not less than transformation temperature, the joint has to be reduced not greater than 98 mm in order to contact to the pipe. Accordingly the joint has to have a shape memory capacity as 4% at a minimum. In result, because of the pipe""s tolerance, a shape memory capacity corresponded to twice of the tolerance may be lost. When a shape memory alloy is enlarged from 100 mm to 101 mm and returns to 100 mm by heating, its shape memory capacity is 1%, and because a circumference of the pipe is in proportion to a diameter, it is possible to calculate a shape memory capacity with a diameter.
Among shape memory alloys developed until now, because NiTi alloy has a shape memory capacity of about 8%, it can sufficiently solve problems due to a tolerance of a pipe. However, because it is too expensive, its application is limited. Cu base alloy has a good memory capacity in theory, however, because a grain is too big, it may be damaged in deformation as 3%, and accordingly it is little used due to bad mechanical characteristics.
In the early 1980"", iron base shape memory alloy such as Fexe2x80x94Mnxe2x80x94Si alloy having a low unit price and a good mechanical characteristic was developed and watched with keen interest, however, it only has a shape memory capacity of about 2%. In order to improve its shape memory capacity, enlarging and heating have to be repeatedly performed and a tolerance of a pipe has to be lowered, etc., and accordingly there are many problems in putting iron base shape memory alloy to practical use.
In addition, a cost related to conveyance and safekeeping has to be considered. A shape memory joint has a strong joining force at a temperature not less than a transformation temperature. In a NiTi alloy joint, a transformation temperature is between xe2x88x9250xc2x0 C.xcx9cxe2x88x92100xc2x0 C., generally it is enlarged at a liquid nitrogen temperature and transported to a factory after being put into liquid nitrogen. After that, two pipes are inserted into the NiTi alloy joint (the joint connects the two pipes), and with temperature rising not less than a transformation temperature, the NiTi alloy joint joins the pipes. In more detail, because the joint has to be kept in liquid nitrogen in order to maintain a temperature of the joint not greater than a transformation temperature, a cost related to alloy storage is too high.
As described above, in the conventional method, there are many problems such as a shape memory capacity loss due to a pipe tolerance, limitation in the kind of applicable shape memory alloy and a cost increase according to enlarged joint storage.
In order to solve the above-mentioned problems, it is an object of the present invention to provide a pipe joining method using shape memory alloy which is capable of preventing a shape memory capacity loss due to a pipe tolerance. In result, shape memory alloy having a small shape memory capacity can be used, and it is possible to join pipes strongly although the pipes have a large tolerance.
In addition, it is another object of the present invention to provide a pipe joining method using shape memory alloy which is capable of eliminating an additional cost occurrence according to conveyance and safekeeping of a processed joint.